HOENBLENDE-SCHISTS, GNEISSES, ETC. OF SARK. 143 



second dyke of mica-trap, half a yard or so thick. The last has a 

 rather rough fracture ; the surface glitters with small scales of 

 mica and shows in parts a curious colour-mottling, which is caused 

 by spots of a purplish brown tint, with rounded outlines more or less 

 connected, like sections of small reniform concretions, the intervals 

 between them being occupied by a dark greenish material. As 

 the rock weathers, the contrast in colour becomes a little more 

 marked, and ultimately the aggregated spots weather out like a 

 roughly pisolitic rock; the diameter of the more globular being 

 about a quarter of an inch. Here and there, however, the struc- 

 ture passes into a more streaky one. 



In these mica-traps ^ the usual brown mica is plentiful, with the 

 crystalline outlines commonly well defined. Another mineral also 

 occurs with a porphyritic habit, the largest and best-defined forms 

 being in the Havre Gosselin rock. The sections indicate that this 

 mineral has belonged to the monoclinic or orthorhombic system, 

 more probably the former. They are defined by a dark line, and 

 occupied by a more or less granular aggregate of secondary 

 minerals. Among these is a carbonate — calcite, or probably a 

 mineral intermediate between this and normal dolomite, besides 

 variable amounts of viridite, chlorite, opacite, and a clear granular 

 mineral, with rather low polarization-tints, not unlike some forms of 

 chalcedony, though possibly one of the zeolitic group ; but in some 

 there is little besides the first named. An altered mineral of this habit 

 is rather common in mica- traps ; probably it has been one of the 

 pyroxene group." In the kersantite of Port du Moulin there is 

 a fair amount of a colourless augite (generally in very bad pre- 

 servation), with some larger irregular-shaped grains of a mineral 

 now consisting of an aggregate of fibres giving bright tints with 

 crossed nicols, but suggestive of the former presence of a rhombic 

 pyroxene. Grains of iron oxide are present in all, and apatite 

 certainly in the first and second. Lath-like crystals of a felspathic. 



^ The Coupee dyke is very much decomposed. We have examined a specimen, 

 from a boulder, which was in a rather better condition, but as even this is by 

 no means well preserved a minute description is needless. There are the 

 usual flakes, more or less regular in form, of brown mica with numerous gran- 

 ules, generally in irregular clots, of iron oxide, in great part limonite. The 

 supposed pyroxenic constituent, mentioned in the other cases, is wanting, unless 

 it be represented by or)e or two irregular spots occupied by aggregates of 

 serpentinous aspect. The present appearance of the rock suggests that it had 

 formerly a glassy base crowded with tiny microliths of plagioclase. The latter 

 can still be traced with more or less distinctness in the granulated brown 

 matrix. The most remarkable feature in the slide is a rather oblong, somewhat 

 rounded spot, about -70" by "45", with fairly definite boundaries, which is con- 

 spicuous from its paler colour and the presence of three larger clots of iron 

 oxide. This is occupied by a matinx like the rest, in which, however, the felspar 

 microliths are rather more distinct, and occasionally show a tendency to a 

 tufted grouping ; there are fewer mica flakes and larger clots of iron oxide, 

 though of the latter there are more disseminated granules. Probably it is an 

 included lump of a slightly less basic variety of the rock. 



- Bonney and Houghton, 'Mica-traps from Keudal, etc' Quart. Jouyn. 

 Geol. Soc. vol. XXXV. (1879) p. 167. The mineral figured by Fouque and Levy 

 (' Mineral. Microgr.' j)l. xxvii.) and referred to bastite, presents some resem- 

 blance to the above-named. 



